A storage-type water heater typically comprises a permanently enclosed water tank, a cylindrical shell coaxial with and radially spaced apart from the water tank to form an annular space between the outer wall of the water tank and the inner wall of the shell, and insulating material in at least a portion of the annular space for providing thermal insulation to the water tank. The water tank has various appurtenances such as inlet, outlet and drain fittings. Additionally, the water heater is provided with a water heating and temperature control system. In an electrical water heater, the water heating and temperature control system includes an electrical resistance heating element. The electrical resistance heating element extends through a fitting in the wall of the water tank such that the heating element is inside the tank. The heating element is connected to an electrical power source outside the water tank. In a gas water heater, the water heating and temperature control system includes a combustion chamber, typically beneath the tank, and a gas heating element (e.g., a gas burner) in the combustion chamber. An air plenum upstream of the combustion chamber provides air to the heating element, and a flue for discharge extends upward from the combustion chamber and through the water tank.
Conventional water heating and temperature control systems typically further include a mechanical thermostat. For the electrical water heater, the mechanical thermostat closes a switch to allow electrical power through the electrical resistance heating element when water in the tank is sensed to be below a selected set-point temperature, and opens the switch to stop electrical power from passing through the electrical resistance heating element when the water in the tank is at or above the set point temperature. Electrical power through the electrical resistance heating element is either fully on, passing full electrical current, or completely off.
Due to variations in manufacture and hysteresis of the mechanical thermostat, the temperature of the water will “overshoot” the desired set-point temperature. In other words, the water heating and temperature control system allows the heating element to continue heating water in the water tank even when the water temperature is above the set point temperature. It would be beneficial to prevent or limit the amount of overshoot of the conventional water heater.